1. Field of the Invention
The present invention generally relates to optical magnetic recording media and storage devices and more particularly, to an optical magnetic recording medium suitable for high density recording and a storage device using the optical magnetic recording medium.
2. Description of the Related Art
An optical magnetic disk employing the magnetically induced super resolution (MSR) method has been suggested as an optical magnetic recording medium by which recording density can be improved. In the MSR method, a multi-layer film of a rare-earth transition metal alloy is used.
FIG. 1 is a cross sectional view of a part of an example of an MSR recording medium of a center aperture detection (CAD) method in a related art. The MSR recording medium shown in FIG. 1 includes a reproducing layer 1, a non-magnetic layer 2, and a recording layer 3. The reproducing layer 1 is made of GdFeCo. The non-magnetic layer 2 is made of SiN. The recording layer 3 is made of TbFeCo. In an MSR reproducing method, a recording bit having a smaller measurement than a spot diameter of a laser beam can be read out by applying a temperature distribution caused by an irradiation of the laser beam to the MSR recording medium. In the CAD method, the reproduction is implemented by using a magnetic field generated from a mark of the recording layer 3. That is, the magnetic field is generated from the mark recorded in the recording layer 3 and the generation is implemented by completing the magnetization of the reproducing layer 1 in a direction of the magnetic field. Namely, the reproduction is implemented by transcribing a magnetic mark of the recording layer 3 to the reproducing layer 1 based on a magneto-static combination. An in-plane layer is used at a room temperature as the reproducing layer 1. The mark of the recording layer 3 with regard to a part having a high temperature based on the irradiation of the laser beam is transcribed so that a part other than the reproduced part is masked. As a result, it is possible to implement the super resolving reproduction.
In addition, the magnetic amplifying magneto-optical system (MAMMOS) in which a medium structure similar to the structure shown in FIG. 1 is used is known. In the MAMMOS, a mark of a recording layer is transcribed to a reproducing layer and the mark transcribed to the reproducing layer transcribed is expanded so that a reproducing signal can be amplified.
In either of the above mentioned methods, a transcription based on the magneto-static combination by the magnetic field from the recording layer is used as a reproduction principle.
In the MSR recording medium of the CAD method as described above, a magnetization of the reproducing layer 1 other than an aperture part is not detected since the magnification is implemented in-plane. Accordingly, there is no signal leaked out from a neighboring track. The MSR recording medium having the CAD method is strong against cross talk and it is possible to make the track pitch narrow. The magnetization of the MSR recording medium of the CAD method is implemented by reversing a direction of the magnetization of the reproducing layer 1 in the magnetic field of the recording layer 3 shown in FIG. 1. In order to obtain a good reproducing property, it is necessary to use a material having a large Ms (magnetizations) as the recording layer 3. Ms of the recording layer 3 can be made large by reducing a rare-earth metal composition such as Tb used for the recording layer 3. Thus, a transcription of the mark recorded in the recording layer 3 to the reproducing layer 1 improves by reducing Tb in the recording layer 3 and thereby increasing Ms. As a result, the MSR recording medium is proof against a neighboring track's influence on the magnetic field from the mark of the reproducing magnetic field.
However, if Tb in the recording layer becomes little, a magnetic field necessary for recording becomes large. Hence, a problem may occur in terms of the magnetic field modulation recording. Accordingly, it is necessary to be capable of recording to the optical magnetic recording medium even if the magnetic field is small enough. In addition, a small recording magnetic field and a small erase magnetic field are preferable in a case of an optical modulation recording. Furthermore, when Tb of the recording layer is little, the reproducing property becomes worse. Particularly, the reproducing property becomes much worse at a short mark. If a property of the short mark is not sufficient, it is not possible to record with a high density. Accordingly, the property at the short mark is very important in order to make the recording capacity of the optical magnetic recording medium large.